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Can Two Bathroom Sinks Share a Drain? (Answered)

Double sinks in a bathroom are absolute blessings for people who are sharing their space, providing enough room for two individuals to get ready at the same time, without any stress. 

To enjoy all the convenience they bring, we might need to address some concerns regarding their drains and vents.

Can two bathroom sinks share a drain?

Two sinks can share the same drain, but it’s not recommended. It may result in blockage and a faulty drainage facility. To avoid such issues, experts suggest installing separate drains and connecting them through the same p-trap. They can also share the same vent following the local plumber’s code.

While two sinks can be connected through the same drain, it is not advised. Shared drains may be the reason why you’ll need to hire a plumber later when things are not draining correctly.

When two sinks share a drain, it may result in blockage and drainage issues. And if such a problem arises with either of the sinks that share the same drain, you will end up with two faulty sinks instead of just one.

On the other hand, two sinks can share the same p-trap without facing any trouble. In fact, it works best when two drains meet at a common p-trap.

With two separate p-traps, one’s air pockets may end up blocking the other’s drain. Positive pressure becomes a major issue in this case. In addition, there will remain a possibility for the growth of slimy microorganisms between the two traps inside the pipe.

When does a double sink need two drains?

In order to understand when and why a double sink may need two drains, we’ll need to break down the mechanism of a sink first. A vanity or double sink includes a countertop placed at the top, supporting the whole structure. It is generally made of hard materials, such as granite.

After that, you’ll find one or two sinks, which are primarily tasked with supplying water. Ready vanities come with the top, the base, and the sinks set up altogether.

However, that’s not the case when you’re building the unit yourself or plumbing the sink. This process offers much more flexibility as it allows you to fix the size yourself, decide whether to merge the drains or keep them separate, decide on the status of the p-trap, and more.

The sink comes with a basin and a drain underneath it. There’s a hole under the sink, which will connect the drain and the tailpipe. It will, in turn, lead to the p-trap.

P-trap is the pipe that connects two drains of 90-degrees with a horizontal pipe and gives the whole structure a shape that looks like the letter P.

To install a unit with two sinks, you should at least have availability of five feet in length between them. This ensures comfortable installation and plumbing.

A double sink needs two drains when the drains are separated, within the same unit or in different units. Both the sinks can meet at the same p-trap if they are placed in the same room.

Can you connect two sinks to one drain, and how?

You can connect two sinks to one drain and upgrade your single vanity to a double vanity. Before you get started, there are a few things that will require your consideration. 

In order to fit two sinks comfortably, you’ll need a double vanity that measures at least six feet in length. Proceed to the following steps only if you can spare that space.

Cap the Water Pipes:

To connect two sinks to one drain, the first thing that you’ll need to do is cap the water lines and begin the plumbing process. 

You can do this by capping off the water supply lines at the shut-off valves or simply by pulling the flexible water supply lines while they’re still connected to the shut-off valves.

Disassemble the Vanity:

After that, remove the countertop and the single sink attached to the sink. Prepare the space where you want to set up the new sink.

Once you have a clean space, begin the installation process. Install the new cabinet as per your needs. You might also want to add new drywall behind the cabinet that will ensure durability.

Install Drain Pipes and Traps:

The next step mainly involves plumbing. Two sinks produce two drains from underneath the countertop. After that, drill holes to fit the drain lines and the hot and cold water supply lines through the cabinet.

Once all the pipes meet at the same spot, prepare a y-branch connection to the new sink. You can use the same p-trap to connect two drains in this case.

Can a tub and sink share the same drain?

A tub and a sink can share the same drain pipe at or up to a point, inside or outside the house. The sharing process is especially beneficial if the two units are placed in the same room. However, they must be connected through strong pipes that are joined well.

The pipes must be installed below the level of both the tub and the sink to ensure smooth operation.  In addition, the pipes must be large enough to withstand the combined flow of water and drainage from the two units. Otherwise, the setup may cause some drainage issues.

Can two sinks share the same P trap?

P-trap is the pipe underneath the counter that connects two or more drain lines together. Two sinks can comfortably share the same p-trap. A sink can share a p-trap with a bathtub as well.

However, you’ll need to abide by the plumber’s code in order to ensure smooth operation. The most common practice is connecting the two sinks to a tee, and later leading them to a single p-trap, which in turn, connects the drains. 

To follow the International Residential Code, make sure that the vertical distance between the sink drain and the p-trap should not exceed 24 inches.

When should a double sink have two traps?

Sinks generally share the same traps for ease of installation and convenient plumbing. However, two traps are installed to solve some specific problems. Separate traps are used for sinks when the pipes from the wall are too high to cover in one installation.

In addition, it is advised to install separate traps when the vertical distance between the drain and the p-trap is higher than 24 inches. Installing one trap in such a case does not follow the plumber’s code, and experts discouraged it.  

Can two sinks share a vent?

Vents are installed to prevent air locks if water ever gets stuck and backs out of the drain. It keeps your sink safe from taking back the dirty water you just drained.

Two sinks can share a vent as long as the floor level rims for the two sinks are the same. That means both the sinks should be at the same height off of the floor.

Ideally, most of the sinks in a room lead to a master vent. It typically goes through the walls and heads outside, often through the roof.

Do you need special plumbing for a double sink?

Double sink demands some special plumbing techniques, as its installation and maintenance differs from that of a regular sink. 

However, you can quickly adapt a single sink’s water supply and drains to install a double sink. All you have to do is incorporate tee fittings, p-traps, special valves, extension pipes, and hoses.

In addition, depending on the plumbing code of your locality, you may have to install separate vents for each sink. They might also need two different drain lines. You can then connect the drain lines of the double sink by using the same p-trap.

Can you replace a single sink with a double?

It is possible to replace a single sink with a double. The entire process can be carried out if you have some time and money to invest and know some plumbing techniques. Double sinks are more practical when you’re sharing a space with someone.

And replacing a single sink with a double can be done as long as there is enough space in the room. The ideal size for a double vanity is six feet. The conversion can be done simply by using p-traps, tee fittings, special valves, and extension pipes.

How much does it cost to turn a single sink into a double sink?

The cost to turn a single sink into a double sink mainly depends on your location and whether you are opting for a premade setup or a customized one. The average cost of installing a double sink is about 1,500 USD.

However, the price may broadly vary depending on the features you’re looking to install. A double sink can cost you something between 500 USD to 4,000 USD. 

In addition, the cost may increase if you need to remove a wall to install the sink, switch the backsplash, or introduce new plumbing setup.

Final thoughts:

Two sinks can share the same drain. However, it may cause drainage issues and blockage in the pipe, so it is discouraged by experts. You can connect the drain pipes through a single p-trap instead. The sinks can also share the same vent adhering to the local and International plumber’s code.